A Free-Space Method for Complex Permittivity Measurement of Bulk and Thin Film Dielectrics at Microwave Frequencies

Abstract

A free-space, non-destructive method for measuring the complex permittivity of a double-layer bulk dielectrics and thin film oxide layers at microwave frequencies have been developed. The method utilizes a spot-focusing antenna system in conjunction with a vector network analyzer in the range of 18-26 GHz. The bulk dielectric was measured using the Transmission Method and Metal-Backed Method, while the Metal-Backed Method was used to investigate the thin films. Both types of samples were sandwiched between two quarter-wavelength Teflon plates to improve the mismatch at the frequencies of measurement. The thin film sample arrangement was backed by an additional metal plate. The double-layer bulk dielectric samples were Teflon-PVC and Plexiglas-PVC, while the thin film samples consisted of SiO 2 layers of different thicknesses grown on doped and undoped Si wafer substrates. The relative permittivity obtained for PVC ranged between 2.62 to 2.93, while those for Plexiglas exhibited values between 2.45 to 2.63. The relative permittivity of SiO 2 deposited on these wafers was between 3.5 to 4.5. All these values are in good agreement with published data. The advantage of the method is its ability to measure the dielectric properties of the films at the midfrequency band irrespective of the substrate type used. Simulations of the measurement setup were carried out using CST Microwave Studio and the simulation results agreed closely with the measurements.

Measurement of Complex Permittivity in Free Space

“…r r r r pro j j (6) Transmission coefficient can be written as multiplication of two exponential functions representing the module and phase [3]: …”

“…Measurement of the complex permittivity in free space is based on placing a sample of unknown material in space between the transmitting and receiving antennas and let a plane electromagnetic wave pass through it [6]. The measurement principle depends on the fact that phase and attenuation of the passing or reflecting wave vary according to the material properties which are defined by the relative complex permittivity which can be expressed as [1] 0 0 i r r (1) where is the relative permittivity or the relative dielectric constant, σ is the conductivity of the sample, is the dielectric constant of free space, ω = 2πf is the angular frequency of the electromagnetic wave.…”

A $Q$ -Band Free-Space Characterization of Carbon Nanotube Composites

“…The thickness of the PTFE specimen is 1.70 mm, while the PMMA specimen has a thickness of about 1.95 mm. The relative dielectric permittivity, ε r of PTFE and PMMA is known to be approximately 1.9 − j10 −4 [42] and 2.4 − j0.015 [43]–[44], respectively.…”

Permittivity measurement of materials of different natures

“…Several experimental methods are used to determine the permittivity and the permeability like: reflection technique [1], transmission technique [2], transmission/reflection technique [3][4]. These methods can be realized in free space [5][6], in a resonant cavity [7], or in a guided structure [8][9]. All these methods are limited by the frequency band or by the nature of material.…”